Unusual Lattice Dynamics and Anisotropic Thermal Conductivity in In2Te5 Due to a Layered Structure and Planar-Coordinated Te-Chains

Searching for novel materials with intrinsically low thermal conductivity and uncovering their origin is an essential way to realize high performance in various energy-related applications, including thermoelectrics. Understanding the role of different structural features in materials can assist us in designing thermal transport properties. Herein, we report thermal transport behavior of In2Te5, which is closely related to its layered zigzag structure and bonding in planar-coordinated Te-chains. The experimental thermal conductivity shows significant anisotropy for the measured two directions and an extremely low value in the interlayer direction of about 0.3 W m–1 K–1 at 673 K. The theoretical calculations of lattice thermal conductivity based on density functional theory support this behavior, and furthermore, they even reveal unprecedented anisotropy in the in-layer direction. We demonstrate that these thermal transport behaviors can be attributed to resonant bonding and lattice dynamics observed at the Te atoms in the planar-coordinated environment. Particularly strikingly, we discover a counterintuitive large positive and negative split of the Born effective charge (+8.7 and −5.1) at the adjacent Te atoms bonded covalently within the same plane. They induce a significant dipole interaction in the x-direction. Optical phonons are significantly softened in the specific direction by these characteristics in the Te-plane, which leads to large anisotropy in thermal transport. Our findings should encourage further excavation of novel features in lattice dynamics out of unexplored materials with complex structures.